1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device using a liquid crystal having a ferroelectric phase (including a ferroelectric liquid crystal and an antiferroelectric liquid crystal) and a method of driving this LCD device. More particularly, this invention relates to an LCD apparatus capable of presenting a gradation display and a method of driving a LCD device in this LCD apparatus.
This invention also relates to an LCD device using a liquid crystal which has a ferroelectric phase and to which dichroic dye is added.
2. Description of the Related Art
A liquid crystal device (FLC-device) using a liquid crystal having a ferroelectric phase is receiving attention due to its higher response and wider view angle than a TN mode LCD device using a nematic liquid crystal.
As an FLC-device, a ferroelectric LCD device using a ferroelectric liquid crystal and an antiferroelectric LCD device using antiferroelectric liquid crystal are known.
Conventionally, for the practical use of an FLC-device, studies have been made on a ferroelectric liquid crystal called an SS-F liquid crystal. But, the FLC-device using an SS-F liquid crystal cannot gradually change the transmittivity and thus cannot present a gradation display.
In this respect, an FLC-device capable of presenting a gradation display has been studied, and it has been proposed to use a ferroelectric liquid crystal whose chiral smectic phase has a helical pitch smaller than the distance between substrates of the display device. This type of ferroelectric liquid crystal is classified to an SBF liquid crystal which has a memory property and a DHF (Deformed Helical Ferroelectric) liquid crystal having no memory property (see "LIQUID CRYSTALS," 1989, Vol. 5, No. 4, pages 1171 to 1177).
In an LCD device using a DHF liquid crystal, this DHF liquid crystal is sealed between substrates, with the helical structure remaining intact. When a voltage whose absolute value is sufficiently large is applied between electrodes facing each other with a liquid crystal layer in between, the DHF liquid crystal becomes either a first alignment state in which the average direction of directions of the long axes of the liquid crystal molecules are aligned substantially to a first alignment direction or a second alignment state in which the average direction of the molecules of the liquid crystal is aligned substantially to a second alignment direction, in accordance with the polarity of the applied voltage. When the absolute value of the applied voltage is lower than the one which sets the DHF liquid crystal to the first alignment state or the second alignment state, the DHF liquid crystal becomes an intermediate alignment state in which the average direction of the long axes of the liquid crystal molecules comes between the first and second alignment directions, due to the helical deformation of the molecule alignment.
In an LCD device using an SBF liquid crystal, this SBF liquid crystal is sealed between substrates, with the helical structure remaining in no electric field state. When a voltage whose absolute value is equal to or greater than a predetermined value is applied between electrodes facing each other with a liquid crystal layer in between, the SBF liquid crystal becomes either a first alignment state in which the directions of the molecules of the liquid crystal are aligned substantially to a first alignment direction or a second alignment state in which the directions of the liquid crystal molecules are aligned substantially to a second alignment direction, in accordance with the polarity of the applied voltage. When the absolute value of the applied voltage is lower than the one which sets the SBF liquid crystal to the first alignment state or the second alignment state, the SBF liquid crystal becomes an intermediate alignment state in which the liquid crystal molecules whose average direction is aligned to the first alignment direction and the liquid crystal molecules whose average direction is aligned to the second alignment direction are mixed.
Conventionally, in an LCD device using a DHF liquid crystal or an SBF liquid crystal, the optical axis of one polarization plate is set parallel to the first or second alignment direction while the optical axis of the other polarization plate is set perpendicular to the optical axis of the former polarization plate.
Even when the voltage corresponding to the gradation to be displayed is applied to the liquid crystal in the LCD devices having the above structures, however, the applied voltage is not associated with the transmittivity of pixels so that the practical level of gradation display cannot be achieved. This is because the hysteresis of the optical characteristics of those LCD devices (the relationships between the applied voltage and the transmittivity) is large. Therefore, even when the voltage corresponding to the display gradation is applied, the display gradation is not specifically set due to the influence of the previously applied voltage.
To control the display gradation by reducing the influence of the hysteresis, a scheme has been proposed which drives the LCD device by applying the voltage that aligns the average direction of the liquid crystal molecules to the first or second alignment direction, and then applying the voltage corresponding to the display gradation. This driving method needs a complicated driving circuit and a longer selection period for writing data in each pixel.
An LCD device using the antiferroelectric liquid crystal (AFLC) displays an image by utilizing the stability of the alignment state of the AFLC. The AFLC has three stable states with regard to the alignment of the liquid crystal molecules. When a voltage equal to or higher than a first threshold value is applied to the AFLC, the AFLC is aligned to a first ferroelectric phase where the liquid crystal molecules are aligned to a first alignment direction or a second ferroelectric phase where the liquid crystal molecules are aligned to a second alignment direction, in accordance with the polarity of the applied voltage. When a voltage whose absolute value is lower than the first threshold value and a second threshold value is applied, the AFLC is aligned to an antiferroelectric phase where the average alignment direction of the liquid crystal molecules is substantially parallel to the normal line of the smectic layer. A pair of polarization plates are located on both side of the LCD device. The transmission axis of the polarization plates are set with the optical axis of the antiferroelectric phase as a reference.
The antiferroelectric liquid crystal has a memory property. More specifically, even when the applied voltage varies within ranges having the first and second thereshold values as their borders, the alignment state of the first or second ferroelectric phase or the antiferroelectric phase is maintained. The conventional antiferroelectric LCD device is driven in a direct matrix manner using this memory property.
The memory property of the AFLC is determined by the difference between the voltage which causes the transition of the liquid crystal to the antiferroelectric phase from the first or second ferroelectric phase and the voltage which causes the transition of the liquid crystal to the first or second ferroelectric phase from the antiferroelectric phase. The greater this voltage difference is, the higher the memory property for memorizing the alignment state becomes.
In this respect, the conventional antiferroelectric LCD device uses a liquid crystal which provides the large voltage difference, as the AFLC.
However, the conventional antiferroelectric LCD device using an AFLC having a higher memory property can hardly control the display gradation and cannot therefore accomplish the gradation display.
Since the conventional FLC-device uses two polarization plates, the amount of light absorption by the polarization plates is large, resulting in a dark display.
Further, the linearly polarized light which has passed the incident-side polarization plate undergoes different birifringence effects for different wavelengths while passing the liquid crystal layer. Therefore, the linearly polarized light becomes different elliptically polarized lights for different wavelengths. The component of each elliptically polarized light which is parallel to the transmission axis of the outgoing-side polarization plate goes out from the transmission axis. Therefore, the intensity of the outgoing light differs wavelength by wavelength, coloring the display.
As the optical characteristic of the LCD device depends on the optical anisotropy .DELTA.n and the product And of the optical anisotropy .DELTA.n and the thickness d of the liquid crystal layer, the liquid crystal and the selection of the liquid crystal layer are limited.